Old Prussian denotes an extinct Baltic language spoken by the inhabitants of the area that later became East Prussia (now in north-eastern Poland, Lithuania and the Kaliningrad oblast of Russia) prior to Polish and German colonization of the area beginning in the 13th century. An experimental community involved in reviving a reconstructed form of the language now exists in the Klaipaedia region of Lithuania.

Old Prussian is closely related to the other extinct western Baltic languages, Galindan (formerly spoken in the territory to the south) and Sudovian (to the east). It is more distantly related to the surviving eastern Baltic languages, Lithuanian and Latvian. The 'Aesti', mentioned by Tacitus in his 'Germania', may have been a people who spoke Old Prussian. Tacitus describes them as being just like the other Suebi (who were a group of Germanic peoples) but with a more Britannic (Celtic) language.

A 16th-century Warmia Prince-Bishop, Marcin Kromer, said the language of the Prussians was totally different from Slavic.

During the Reformation and thereafter other groups of people from Poland, Lithuania, France, Austria also found refuge in Prussia. These new immigrants also caused a slow decline in the use of Old Prussian as Prussians began to adopt the languages of the newcomers. Old Prussian probably ceased to be spoken around the end of the 17th century with the great plague.

It is called "Old Prussian" to avoid confusion through the adjective "Prussian", which relates also to the later German state. Old Prussian began to be written down in about the 14th century. A small amount of literature in the Old Prussian language survives.